Centrifugal compressors are utilized extensively in many industries today across a wide variety of applications. A consistent request from users of centrifugal compressors to the manufactures of centrifugal compressors is to produce a machine with a smaller size and lower cost having the same performance characteristics of the existing generation of centrifugal compressors. This request involves the necessity of improving the efficiency of the compressor such that reducing the size of the centrifugal compressor results in a lower cost machine without reducing the performance of the machine.
Centrifugal compressors generally have multiple stages, each comprising a rotating impeller, and return channels which include fixed return channel blades forming fixed vanes for redirecting the compressed gas from the exit location of the impeller of one stage to the entry location of the impeller of the next stage along the gas flow path through the machine and for removing the tangential component of the flow.
In some known centrifugal compressors movable inlet guide vanes, also named variable inlet guide vanes, are provided for modifying the flow conditions of the incoming gaseous flow depending upon the operating conditions of the machine.
FIG. 1 illustrates a multistage centrifugal compressor of the current art, globally labeled 100. The compressor 100 has an outer casing 101 provided with an inlet manifold 102 and an outlet manifold 103. Inside the casing 101 several components, globally named “compressor bundle”, are arranged, which define a plurality of compressor stages. More specifically, inside the casing 1 a rotary shaft 105 is arranged. The shaft 105 is supported by two end bearings 106, 107. Each bearing can in actual fact be a bearing assembly comprised of one or more bearing components. Between the two bearings 106, 107 a plurality of impellers 109 are mounted on the shaft 105. The inlet 109A of the first impeller 109 is in fluid communication with an inlet plenum 111, wherein gas to be compressed is delivered through the inlet manifold 102. The gas flow enters the inlet plenum 111 radially and is then delivered through a set of movable inlet guide vanes 113 and enters the first impeller 109 in a substantially axial direction.
The outlet 109B of the last impeller 109 is in fluid communication with a volute 115, which collects the compressed gas and delivers it towards the outlet manifold 103.
Stationary diaphragms 117 are arranged between each pair of sequentially arranged impellers 109. Diaphragms 117 can be formed as separate, axially stacked components. In other embodiments, the diaphragms 117 can be formed in two substantially symmetrical halves. Each diaphragm 117 defines return channels 119 which extend from the radial outlet of the respective upstream impeller 109 to the inlet of the respective downstream impeller 109, returning the compressed gaseous flow from the upstream impeller towards the downstream impeller. Fixed blades 121 are provided in the return channels 119, for removing the tangential component of the flow while redirecting the compressed gas from the upstream to the downstream impeller.